problem on condenser / darkfield ?

[dorianxes]

Hello,
it seems i don't understand how it works, my condenser is bad, i've missprinted...
I'll try to explain my problem as accurate as possible
(i use dust just for the example)

My setup : SW380T and Swiftcamera 5MP-usb3

first of all i've redrawn them on C4d then 3d-printed :




then I tried on my condenser :

4x
7.7mm
works fine


10x
7.7mm
works fine


and then
40x
7.7mm
doesn't work anymore (of course)


My problem is only the 7.7mm works on 4 and 10x
other sizes does'nt work at all
I have full black or some kind of brown spot in the middle.

I also tried to move up and down my condenser but nothing improves.

Where did i went wrong ?

Thanks

Bonus
4x
13.5mm
(full black :/)

[Hobbyst46]

I believe that there is nothing wrong with what you did. Those stops are easier to fit to the low NA objectives. The relatively high NA (0.65 ?) of the 40X requires very precise tailoring of a hollow light cone from the condenser - optimal distance of the central opaque disc from the specimen, optimal diameter of the disc, precise centration of the disc, and - very important - very bright illumination. Most folks post nice DF from lower NA objectives. The 40X will likely work with a dedicated condenser.

[dorianxes]

Oh ok thanks.
But what is the purpose of the largests ones ?

[HowardHopkinson]

I too have the swift SW380T and a set of patch stops and yes, I have exactly the same problem with the 40X objective.
I have found that if I carefully swing out the filter holder, I can in fact get an approximation of darkfield with various patch stops.
With the 40X objective just try experimenting with the patch size and the filter holder position.
I have yet to try altering the condenser position but will definitely give it a go and see what happens.

[Hobbyst46]

Oh ok thanks.
But what is the purpose of the largests ones ?

Because of the difficulty of tailoring a stop disc to an objective, the trend is to sell, or at least recommend to fabricate, a "set" of stops, that cover a range of diameters, hoping that one of them will somehow fit the objective. Larger diameters for higher NA objectives. It is a trial and error approach, that IMHO seldom works for 40X objectives and up. My guess is that the specific diameters on your list had been suggested by someone who found they work on his microscope.

[dorianxes]

Ok i undertand it better now. Thanks

[MickH]

Hello Howard, did you get your stops from eBay or print your own? I've found someone on eBay who, for around £12 each set, prints and sells sets of stops which simulate phase contrast and dark field.

I've included a link here to a Blog where someone tests these filters and mentions (no real detail) adjusting the condenser position and diaphragm for optimum results. It seems to be a suck-it-and-see approach that, presumably, varies from microscope to microscope and filter to filter.

Here's the Blog post: https://microscopyfiltersbypedropires.blogspot.com/ and for clarity I have no connection with either the eBay seller or the Blogger.

PS. I don't think I've ever been a member of a forum where so many members have access to 3D printers!

Cheers

[HowardHopkinson]

Yes, I got the patch stops from eBay from the same vendor as in the article you linked to, which is very interesting indeed. These are the ones I got.
https://www.ebay.co.uk/itm/Microscope-D ... 2749.l2648

9 filters in a set.
The next time I have the microscope out, I will experiment with the 40X objective and condenser height.

[HowardHopkinson]

As promised I did try lowering the condenser when using the 40X objective with the patch stops. I have to report that it didn't help and in fact made the issue we have worse. I guess I'll just continue adjusting the filter holder to get a kind of darkfield/oblique view.

[apochronaut]

There has been a veritable industry established around patch stops and Rheinberg filters over the past few years. Nice to see simple and straightforward ways of mucking around with contrast choices.
To add a bit to what Doron mentioned, problems with DF are 4-fold or at least most of the problems are one or more of four.. Too light, too dark, no image at all and low resolution.
Too light is caused by the objective N.A. being above a certain threshold relative to the condenser N.A.. Usually, a minimum of .20 N.A. differential must exist between the objective N.A. and the condenser N.A.
Too dark. Where an image exists but it is very dark, the culprit is that not enough light is getting to the subject. While that seems pretty obvious there are three versions of that problem. When the contrast and the brightness are low, the stop is too large, only allowing a narrow cone of light through. If the illumination is low but the contrast not too bad the culprit might be the slide thickness. DF requires absolute precision, moreso as Doron pointed out with higher N.A. objectives. The hollow cone/slide thickness interplay becomes very touchy. Typically, with DF condensers the slide thickness is specified within a range. I can only assume this to be also true with modified abbe condensers too. I don't use one because oil DF condensers are so nice to use but the principal is the same. It all hinges on the angle of the light incident on the bottom if the slide, striking the top surface of the slide at an acute enough angle to cause total internal reflection. The slide must be the correct thickness. A third possibility but usually only with high magnification objectives is that the illuminator is too weak. That normally doesn't occur with objectives below 70 or 80X.
No Image. Most likely caused by the stop being too large. It doesn't normally happen with DF condensers.
Poor resolution. Poorly centered or the condenser working at too low an N.A. Oil immersion condensers should be oiled.

Condensers I have seen that have manufactured add on DF stops that work well for higher N.A. have the stops applied on the top of the upper lens, not in a filter tray.

[Placozoa]

I had this problem also, but I fixed it. It was caused by the darkfield patch stop not being in focus. The diaphragm is normally in focus and not the filter, so adding the darkfield stop to the filter rack does not work for high magnifications. So, what I did was move the darkfield stop down below the lens that focuses the light before it gets to the diaphragm. I am lazy, so I simply used a piece of clear plastic for modelling, cut into a strip, with the darkfield stop taped to the strip. Then I can slide the strip to add the stop or remove it. It should be lower in the light path and larger and I would get an even better result, but hey, it works fine as is. I have good darkfield for 60x, 40x, and 10x objectives with a bright ring around the very edge on 4x.
I suspect this is why expen$ive darkfield is done with a darkfield condenser and not a filter stop.

[Greg Howald]

1. Try closing in a bit with the iris diaphragm at 40x.
2. Is it possible to center the condenser on your scope?
3. I had the same problem with my similar Amscope 420. It is not designed for accepting a dark field condenser but I bought a dark field condenser that would fit into the condenser holder, took it apart and through trial and error was able to make a stop of the right size. I also ordered an iris diaphragm and made a field diaphragm for it to help with contrast control. In this way I come very close to true dark field images at 40x.
Greg